The present invention relates to a method and a device for correcting a signal, and more particularly to a method and a device for correcting an analog signal outputted from a partial response channel to target levels.
Optical disks such as compact disks (CDs), video compact disks (VCDs) and digital versatile disk (DVDs) are played by recording and reproducing devices such as VCD or DVD players. In a typical digital data recording and reproducing system of FIG. 1, a digital data sequence u is encoded by an error control encoder 11 and then modulated by a modulator 12 so as to be modified as a recording signal x. The recording signal x is suitable to be written into a digital data recording medium 10 by means of a write-in device 13, and read out by a pickup head 14. The signal read by the pickup head 14 is processed by an equalizer 15 into a signal y, and then the equalized signal y is processed by a detector 16 according to a sequential maximum likelihood algorithm into a read-out signal xxe2x80x2 in the same format as that of the recording signal x. The maximum likelihood algorithm, which is usually implemented as a Viterbi decoder, is well known in the art and need not be further described in detail herein. The read-out signal xxe2x80x2 is subsequently demodulated and decoded by a demodulator 17 and an error control decoder 18, respectively, so as to obtain a recovered data sequence uxe2x80x2. Generally, the equalizer 15, the detector 16, the demodulator 17 and the error control decoder 18 are incorporated in a control chip of an optical disk drive.
The procedures for converting the recording signal x into the signal y will be illustrated in reference to FIGS. 2 and 3. In FIG. 2, the means for processing the recording signal x prior to entering the equalizer 15 can be simplified as a channel 20 The transfer function between the input/output signals x and z of the channel 20 is referred to as Z(D)/X(D)=1+a1*D+a2*D2+a3*D3+a4*D4+ . . . +anxe2x88x921*Dnxe2x88x921 in a form of polynomial, where D is a delay time. The equalizer 15 is employed to remove some items of higher power in the polynomial, and thus the channel 20 and the equalizer 15 are viewed as an integrated partial response channel. For this partial response channel, a transfer function, for example PR(1,1), PR(1,2,1) or PR(1,1,1,1), can be applied for the transfer from the signal x to the signal y. The transfer function PR(1,1) indicates Y(D)/X(D)=1+D with target levels of xe2x88x921, 0 and 1. The transfer function PR(1,2,1) indicates Y(D)/X(D)=1+2*D+D2 with target levels of xe2x88x922, xe2x88x921, 1 and 2. The transfer function PR(1,1,1,1) indicates Y(D)/X(D)=1+D+D2+D3 with target levels of xe2x88x922, xe2x88x921, 0, 1, 2.
The function PR(1,1) is unsatisfactory for being applied to real products because the noise cannot be effectively filtered out. Although the function PR(1,1,1,1) can result in good performance of the partial response channel, the cost thereof is relatively high. Thus, the function PR(1,2,1) is discussed hereinafter.
FIGS. 3(a) to 3(c) are timing waveform diagrams illustrating the corresponding signals processed in the partial response channel based on the transfer function PR(1,2,1). As shown in FIG. 3(a), the recording signal x consists essentially of levels 0.5 and +0.5. The ideal waveform of the signal y, i.e. yid, after being processed by the partial response channel on the basis of the transfer function PR(1,2,1), i.e. Y(D)/X(D)=1+2*D+D2, is shown in FIG. 3(b). The signal is supposed to be distributed at target levels of xe2x88x922, 1, 1 and 2. However, the waveform of the signal y is practically somewhat drifted from the ideal target levels due to the mismatch between ideal partial response channel and real one. The real waveform of the signal y. i.e. yreal, can for example be seen in FIG. 3(c).
As shown, in spite of precise location on the four target levels xe2x88x922, xe2x88x921, 1, 2 for most of the sampled points, some sampled points are drifted from the target levels. Particularly for three sequential sampled points respectively in response to three sampling cycles, which are so-called as 3T sampled points and for example include sampled points a, b and c as shown in FIG. 3(c), the middle sampled point b is possibly greatly deviated from its target level xe2x88x922 due to the significant variation during a short period of time. Under this circumstance, a mismatch problem occurs. When the mismatched signal y from the partial response channel is sent to the detector 16, the read-out signal xxe2x80x2 may not be recovered to its original state as the recording signal x by a maximum likelihood algorithm.
It is an object of the present invention to provide a method and a device for correcting an analog signal from a partial response channel, which effectively locate three sequential and then correct the middle sampled point to a target level.
In accordance with an aspect of the present invention, there is provided a method for correcting an analog signal to target levels. The analog signal is transmitted from a partial response channel and comprises a plurality of periodically sampled points. The method for correcting an analog signal to target levels comprising steps of picking up three sequentially sampled points according to a specified criterion, and adjusting a middle one of the three sequentially sampled points to one of the target levels.
In an embodiment, the analog signal is a radio frequency (RF) signal.
In an embodiment, the analog signal is obtained by Y(D)=X(D)*(1 +2*D+D2), where D is a delay time, and X(D) is an input of the partial response channel. The analog signal is to be corrected into four target levels xe2x88x922, xe2x88x921, 1 and 2.
In an embodiment, the three sequentially sampled points have respective levels less than a threshold value, immediately follow one sampled point having a level greater than the threshold value, and are followed by one sampled point having a level greater than the threshold value. The middle one of the three sequentially sampled points is adjusted to a smallest one of the target levels. Preferably, the threshold value is xe2x80x9c0xe2x80x9d, and the smallest target level is xe2x80x9cxe2x88x922xe2x80x9d.
Alternatively, the three sequentially sampled points have respective levels greater than a threshold value, immediately follow one sampled point having a level less than the threshold value, and are followed by one sampled point having a level less than the threshold value. The middle one of the three sequentially sampled points is adjusted to a largest one of the target levels. Preferably, the threshold value is xe2x80x9c0xe2x80x9d, and the largest target level is xe2x80x9c2xe2x80x9d.
In accordance with another aspect of the present invention, there is provided a method for correcting an analog signal to target levels. Firstly, the analog signal is periodically sampled to obtain a plurality of sampled points. Then, levels of the sampled points are compared with a threshold value to find a set of sequentially sampled points including a head and a tail ones, each having a first comparing result with the threshold value, and the other intermediate ones, each having a second comparing result with the threshold value. Then, one of the set of sequentially sampled points, which has the second comparing result with the threshold value, is adjusted to one of the target levels.
In an embodiment, the set of sequentially sampled points includes five consecutive sampled points.
In an embodiment, the first comparing result indicates that the level of each of the head and tail sampled points is greater than the threshold value, and the second comparing result indicates that the level of each of the intermediate sampled points is less than the threshold value. The step of adjusting one of the set of sequentially sampled points is performed by adjusting a middle one of the intermediate sampled points to a smallest one of the target levels.
Alternatively, the first comparing result indicates that the level of each of the head and tail sampled points is less than the threshold value, and the second comparing result indicates that the level of each of the intermediate sampled points is greater than the threshold value. The step of adjusting one of the set of sequentially sampled points is performed by adjusting a middle one of the intermediate sampled points to a largest one of the target levels.
In accordance with another aspect of the present invention, there is provided a device for correcting an analog signal into target levels for use with a partial response channel. The device comprises a delay unit, a first comparator, a second comparator and a correcting circuit. The delay unit includes a head delay element, a plurality of intermediate delay elements and a tail delay element electrically connected in series, and each receiving the analog signal and delaying sampled points by a certain time period. The first comparator is in communication with the head and the intermediate delay elements for comparing levels of first sampled points outputted by the head and the intermediate delay elements with a threshold value, and outputting a first triggering signal in response to a first comparing result. The second comparator is in communication with the partial response channel and the tail delay element for comparing levels of second sampled points outputted by the partial response channel and the tail delay element with a threshold value, and outputting a second triggering signal in response to a second comparing result. The correcting circuit is in communication with a middle one of the intermediate delay elements, the first comparator and the second comparator for adjusting the sampled point outputted by the middle one of the intermediate delay elements to one of the target levels in response to the simultaneous occurrence of the first and the second triggering signals.
In an embodiment, the delay unit includes four delay elements, and the correcting circuit is electrically connected to a second one of the five delay elements.
In an embodiment, the first comparing result indicates that the level of each of the first sampled points is greater than the threshold value, and the second comparing result indicates that the level of each of the second sampled points is less than the threshold value. The correcting circuit adjusts the sampled point outputted by the middle one of the intermediate delay elements to a smallest one of the target levels. Preferably, the threshold value is xe2x80x9c0xe2x80x9d, and the target level is xe2x80x9cxe2x88x922xe2x80x9d. Alternatively, the first comparing result indicates that the level of each of the first sampled points is less than the threshold value, and the second comparing result indicates that the level of each of the second sampled points is greater than the threshold value. The correcting circuit adjusts the point outputted by the middle one of the intermediate delay elements to one of the target levels. Preferably, the threshold value is xe2x80x9c0xe2x80x9d, and the target level is xe2x80x9c2xe2x80x9d.
The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: